1. Field of the Invention
The present invention relates to a control of vehicle automatic transmissions and more particularly to shift control means for a transmission gear mechanism.
2. Description of Prior Art
Conventional automatic transmissions for motor vehicles include a hydraulic torque converter and a multiple stage transmission gear mechanism connected with the torque converter. The torque converter has an automatic torque changing function wherein a torque amplification is made when the speed of the turbine runner is lower than that of the pump impeller, however, since the torque ratio is by itself insufficient for a normal vehicle operation, the multiple stage transmission gear mechanism is provided and its gear stage is appropriately selected in accordance with the vehicle operating conditions. For the purpose, modern automatic transmissions are equipped with microprocessors which include gear shift maps for determining engine operating conditions at which the transmission gear mechanisms are shifted up or down.
Usually, the shift maps are so determined that an appropriate gear stage is selected to ensure an appropriate capability of acceleration under a normal operation on a normal road. It should however be noted that the conventional control has not been satisfactory under operating conditions other than the normal operation. For example, under an uphill operation, the selected gear stage often becomes inappropriate to provide a satisfactory driving torque. In view of the above problem, there is proposed by Japanese patent application No. 54-111928 filed on Sept. 1, 1979 and disclosed for public inspection on Apr. 15, 1981 under the disclosure number of 56-39353 to use under an uphill operation gear shift maps which are modified for such uphill operation. The proposed control system is considered as being capable of solving problems of insufficient driving torque under the uphill operation, however, it does not consider anything about fuel economy which may become bad due to a case of inappropriate gear shift maps. In city area operations, for example, vehicle is required to repeat frequently stop and start due to traffic jams and, under such circumstances, the transmission gear mechanism takes the first gear stage irrespective of a very small acceleration resulting in a very poor fuel economy.